A plating film or the like is formed on a metal base material by plating, vapor deposition or the like to impart, to the metal base material, capability such as corrosion resistance, heat resistance, water repellency or hydrophilicity, as the case may be. However, in the case where a plating film or the like is formed on the metal base material, when an oxide film has been formed on the metal base material, the plating film or the like sometimes tends to be peeled off the metal base material.
The oxide film is formed by a heat treatment or a chemical treatment with an oxidizing agent or the like of the metal base material. In particular, on a metal base material that includes an Fe component such as stainless steel such as SUS304 or SUS316, or iron, an iron-based oxide film or the like is readily formed by heat treatment or the like. For example, stainless steel or the like is used for the base material of a separator for use in a fuel cell. Heat treatment is performed at the time of forming, resin coating or similar processing of a separator for use in a fuel cell, and thus an oxide film may be formed on the stainless steel base material of the separator for use in a fuel cell. Additionally, for the purpose of imparting corrosion resistance or the like to the separator for use in a fuel cell, a plating treatment or the like may be applied onto the stainless steel of the separator for use in a fuel cell, having been subjected to the above-described heat treatment.
An iron-based oxide film or the like is formed on the stainless steel base material having been subjected to the heat treatment, and hence, even when plating is applied to form a plating film on the stainless steel base material, the adhesion of the plating film is poor and thus the plating film tends to peel off due to the effect of the iron-based oxide film.
For example, JP 7-303977 A has proposed a method of surface treatment of stainless steel in which method, for the purpose of removing the oxide film on the stainless steel, a reduction treatment based on an electrolysis treatment is applied to the stainless steel having the oxide film.
Additionally, for example, JP 2-85394 A has proposed a method of surface treatment of stainless steel in which method, for the purpose of removing the oxide film on the stainless steel, an oxidation treatment based on an electrolysis treatment is applied to the stainless steel having the oxide film.
Additionally, for example, JP 7-188976 A has proposed a method of surface treatment of stainless steel in which method, for the purpose of removing the oxide film on the stainless steel, an oxidation treatment based on an electrolysis treatment is applied to the stainless steel having the oxide film. and thereafter a reduction treatment based on an electrolysis treatment is applied to the stainless steel.
The main component of the oxide film on stainless steel is Fe2O3. Only with a reduction treatment, as is the case in the method of surface treatment of stainless steel according to JP 7-303977 A, Fe2O3 is reduced into FeO, but it is extremely difficult to remove the oxide film from on stainless steel.
Additionally, with an oxidation treatment, it is difficult to remove Fe2O3, as is the case in the method of surface treatment of stainless steel in JP 2-85394 A, and an attempt to remove Fe2O3 only with an oxidation treatment may result in erosion of the stainless steel that is the base material.
Additionally, it is difficult, as described above, to remove Fe2O3 even when an oxidation treatment is first performed, and consequently it is difficult to remove the oxide film from on the stainless steel even when a reduction treatment is subsequently performed, as is the case in the method of surface treatment of stainless steel in JP 7-188976 A.